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The oxidation and H pick-up behaviour of Zr alloys used in the nuclear industry are dictated by alloying additions and coatings. This thesis presents computational findings on the kinetics of alloying element diffusion, formation and stability of secondary phases, and H trapping behaviour in these precipitates. In particular, it focuses on phases found in Fe- and Cr-containing Zr alloys, Nb-containing Zr alloys and Cr-coated Zr alloys. Density functional theory calculations were carried out to calculate the solubility of H in Zr(Fe,Cr)2 solid solutions with compositions representative of second phase particles found in Zircaloy-2 and Zircaloy-4. The results show that Zr(Fe,Cr)2 second phase particles are not strong traps for H. Combining these findings with experimental work carried out by colleagues, it is proposed that the interface between second phase particles and HCP-Zr matrix may act as possible trapping sites for H in Fe and Cr containing alloys. In Zr-Nb alloys, the solubility of H in β-Zr and β-Nb phases was calculated. It is found that the solubility of H increases with increasing Zr content in these β phases with H more soluble in β-Zr than in β-Nb. This explains the reported increase in terminal solid solubility of H in the presence of β-Zr in Zr-Nb alloys. A novel model is also presented, which predicts the solubility of H, to a remarkable level of accuracy, at any composition of (Zr,Nb) solid solution at a fraction of the computational cost required by current techniques. The transferability of the model to other systems and the inclusion of radiation-induced defects in the model are also discussed. Finally, the microstructural evolution of Cr-coated Zr alloy cladding is studied by calculating the vacancy-mediated diffusion of Zr and Nb solutes in BCC-Cr as well as Zr and Cr solutes in BCC-Nb. In BCC-Cr, both Nb and Zr are faster diffusers than Cr self-diffusion. Both Zr and Nb segregate towards vacancy sinks in BCC-Cr at normal reactor operating temperatures, but at elevated temperatures their flux is expected to be in opposite directions. In BCC-Nb, Cr is a slower diffuser than Nb self-diffusion, while Zr is faster; and both Zr and Cr are expected to decorate vacancy sinks in BCC-Nb at all relevant temperatures. The implications of these findings for Cr-coated Zr alloy cladding are discussed. The findings of this thesis can help the industry design alloys with lower H pick-up, allowing increased utilisation of nuclear fuel inside a reactor. |
